Long and Small Fatigue Crack Growth in Aluminum Alloys

Fatigue crack growth (FCG) studies at various stress ratios (R=0.1, 0.5, 0.7) were performed on solution-strengthened (cast A535) and precipitation-strengthened (cast A356, 319, A390 and wrought 6061) aluminum alloys. Microstructures were altered through processing, chemistry, and heat treatment (T4, T6, T7) to shed light on the effects of various intrinsic material characteristic features on FCG (e.g. Si amount/type/morphology, grain size, secondary dendrite arm spacing, precipitate type/size). In this context, mechanisms of long and small fatigue crack growth at the microstructural scale of the studied alloys were identified, and loading-microstructure-damage mechanisms design maps were created. The differences in FCG responses between long, physically-small, and microstructurally-small cracks were systematically evaluated, and an original fracture mechanics — materials science combined model that accounts for these differences was developed, having both material and crack size dependency. Examples of the use of this integrated methodology for design and fatigue life predictions will also be given.